The invention relates to rolls for use in calenders and like machines, and more particularly to improvements in systems of rolls wherein one or more rolls have deformable hollow cylindrical shells and are rotatable with radial play about stationary carriers with the interposition of preferably hydrostatic supporting elements. Still more particularly, the invention relates to improvements in systems of rolls wherein the supporting elements include a row of primary supporting elements adjacent the nip which the deformable shell defines with an adjacent roll, and a row of secondary supporting elements substantially diametrically opposite the row of primary supporting elements.
Commonly owned German Offenlegungsschrift No. 33 25 385 discloses a roll wherein the primary supporting elements are identical with the secondary supporting elements and wherein the pressure generating devices (such as cylinder and piston units) for the primary supporting elements are identical with the pressure generating devices for the secondary supporting elements. Each supporting element has an exposed surface which is adjacent the internal surface of the shell and has a pocket which receives hydraulic fluid from the associated pressure generating device by way of a throttling channel. The pressure of hydraulic fluid which is being admitted into the pockets of the supporting elements is variable to thereby alter the profile of the shell and to regulate the width of selected portions of the nip between the shell and the periphery of the adjacent roll. The films of hydraulic fluid between the internal surface of the shell and the surfaces of the supporting elements reduce friction to a value well below that which would develop in response to the establishment of direct mechanical contact between the shell and the supporting elements.
The quality of the web (e.g., a continuous strip of paper, metal or plastic) which is treated while running through the nip of the shell and the adjacent roll depends to a large extent upon the accuracy of regulation of pressure in the pockets of the supporting elements and in the pressure generating devices. The force with which the primary and secondary supporting elements are urged toward the internal surface of the shell determines the dimensions of the nip as well as the force which is applied to the running web. In addition, the rows of primary and secondary supporting elements (such rows are disposed substantially diametrically opposite each other) exert a pronounced stabilizing influence by opposing lateral movements of the shell.
The aforedescribed roll operates quite satisfactorily except that the shell is often heated to an elevated temperature and that it is not readily possible to maintain each and every portion (especially the end portions) of the roll at a desired distance from the respective portions of the adjacent roll or rolls.
Localized heating of the shell forming part of the roll which is disclosed in the Offenlegungsschrift is attributable to randomly occurring friction between the secondary supporting elements and the internal surface of the shell. The force with which the primary supporting elements urge the shell toward the adjacent roll is greater than the force with which the secondary supporting elements urge the shell away from such roll. This is deemed necessary in order to ensure that the nip of the cooperating rolls will have a desired width and that the material running through the nip will be subjected to a predictable calendering or other treatment. The magnitude of the forces is regulated by regulating the pressure of hydraulic fluid in the pockets of the primary and secondary supporting elements. Since the dimensions of convex surfaces of the primary supporting elements are the same as those of the convex surfaces of the secondary supporting elements, and the dimensions of pockets in all of the convex surfaces are also the same, the pressure of fluid in the pockets of the secondary supporting elements must be less than the pressure in the pockets of the primary supporting elements. It can happen that the pressure in the pockets of the primary supporting elements is too low to enable the primary supporting elements to act as hydrostatic bearings, i.e., the liquid films between the convex surfaces of the secondary supporting elements and the internal surface of the shell are too thin so that portions of or the entire convex surfaces of the secondary supporting elements come in direct (metal-to-metal) contact with the shell. One can speak of mixed friction including hydraulic and metallic friction. Such mixed friction is more pronounced than a pure hydraulic friction and leads to unexpected and undesirable localized heating of the shell.
Somewhat similar rolls are disclosed in German Offenlegungsschrift No. 30 49 080 and in German Auslegeschrift No. 26 32 452.